1. Field of the Invention
This invention relates to a power converter that interchanges power between a first A.C. system and a second A.C. system.
2. Description of the Background
The construction of known power converter of the above-noted type, shown in FIG. 10 includes a main circuit in which the D.C. sides in each case of an externally commutated converter 5A that performs a A.C. to D.C. converter action and an externally commutated converter 5B that performs a D.C. to A.C. converter action are connected through D.C. reactors 4A and 4B and a D.C. line 8, while the A.C. sides in each case of externally commutated converters 5A and 5B are connected through respective converter transformers 3A and 3B and circuit breakers 2A and 2B to A.C. systems 1A and 1B. Externally commutated converters 5A and 5B are constructed for example as shown in FIG. 11 by connecting six thyristors 5U, 5V, 5W, 5X, 5Y, and 5Z in six arms in a three-phase bridge arrangement.
The control device that controls externally commutated converters 5A and 5B consists of an interchange power control system provided for converter 5A and a margin angle control system provided for converter 5B, their functions being respectively allocated to these two externally commutated converters.
The interchange power control system is equipped with an interchange power control circuit (APC) 41 that outputs a D.C. current reference value I.sub.dp that is necessary to result in an active interchange power P.sub.d detected by an active power detector 22. Detector 22 determines interchange power from the voltage and current extracted through a potential transformer 13 and A.C. transformer 14 on the system side of the transformer 3B on the side of converter 5B whereby it can be determined whether the detected interchange power coincides with the interchange power reference value P.sub.dp set by an interchange power setting device 31. The actual D.C. current I.sub.d of D.C. line 8 is detected by D.C. current transformer 11A and D.C. current detector 21A. Converter 5A is controlled by means of D.C. current control circuit (ACC) 42, phase control circuit (PHS) 43A and pulse amplification circuit (PA) 44A so that current I.sub.d of D.C. line 8 coincides with D.C. current reference value I.sub.dp mentioned above.
A specific example of the construction of interchange power control circuit 41 is shown in FIG. 12. In the interchange power control circuit 41 of FIG. 12, the deviation between the active detected power value P.sub.d found by active power detector 22 and the interchange reference power value P.sub.dp set by interchange power setting device 31 is found by adder 412, amplified by interchange power regulator (APR) 411, and the D.C. current reference value I.sub.dp for controlling the interchange power is thereby formed. It should be noted that the D.C. voltage in a power converter of this type is held practically constant, so using the D.C. voltage reference value E.sub.dp corresponding to steady operation, as shown in interchange power control circuit 41A of FIG. 13, the calculation P.sub.dp /E.sub.dp consisting of dividing the interchange power reference value P.sub.dp as set by interchange power setting device 31 by the D.C. voltage reference value E.sub.dp as set by D.C. voltage setting device 32 is performed by divider 413. The D.C. current reference value I.sub.dp is created by adding the output signal of divider 413 to the output signal of interchange power regulator 411, using adder 414.
A specific example of the construction of the D.C. current control circuit 42 is shown in FIG. 14. As can be there seen, in the control circuit 42, the deviation between the D.C. current detected value I.sub.p found by the D.C. current detector 21A and the D.C. current reference value I.sub.dp produced by the interchange power control circuit 41 is determined by adder 422. This deviation is amplified by D.C. current regulator (ACR) 421, and its output signal is fed to phase control circuit 43A.
Phase control circuit 43A controls the firing timing of converter 5A by means of pulse amplification circuit 44A, as already described, by determining the control delay angle of externally commutated converter 5A in accordance with the output signal of D.C. current control circuit 42.
The margin angle control system controls the firing timing of converter 5B by means of pulse amplification circuit 44B, by determining control advance angle .gamma. from a margin angle control circuit (A.gamma.C) 45 and phase control circuit 43B. such as to maintain a margin angle, in order to avoid failure of commutation of externally commutated converter 5B that carries out the D.C. to a A.C. conversion action. The construction of margin angle control circuit 45 itself may be, for example, as described in published Japanese patent publication No. 46956/1983, so a detailed description is omitted here.
With the above construction, the D.C. voltage of the power converter device is determined with a margin angle being maintained at externally commutated converter 5B, and stable operation of the power converter device is continued by controlling the D.C. current at externally commutated converter 5A for interchange power control. This method of control is disclosed in, for example, Japanese patent application laid-open No. 107443/1975.
In the conventional device constructed as above-described, even though the interchange power between the two A.C. system could be controlled, the reactive power could not be controlled. Or, if the reactive power was to be controlled, the construction became difficult and complicated.